1) Field of the Invention
The present invention relates to a gradation correcting apparatus and specifically to a gradation correcting apparatus for use in television receivers, video tape recorders, video projectors, and the like. In particular a gain control circuit for controlling the quantity of correction applied an input image in order to automatically adjust that image is disclosed.
2) Description of the Prior Art
A gradation correcting apparatus automatically acts on a video gradation signal (which includes information such as picture brightness and picture contrast) to obtain a television picture with optimum contrast. With the recent trend towards high quality television receivers, such devices have received much attention.
A gradation correcting apparatus typically includes a gain control circuit in order to control correction quantity.
A conventional gradation correcting apparatus, including a gain control circuit, is explained with reference to the block diagram shown in FIG. 1.
The gradation correcting apparatus comprises: a histogram memory 5 for representing brightness distribution of an input brightness signal; a look up table (LUT) arithmetic circuit 6 for performing accumulative addition of frequency in the histogram, and thereafter normalizing each accumulative frequency as described later; a look up table memory 7 for storing the normalized accumulative frequency supplied from the LUT arithmetic circuit 6 and supplying a correction quantity in response to the brightness level of the input brightness signal; a maximum value detecting circuit 8 for detecting the maximum value of difference between adjacent normalized accumulative frequencies in the LUT arithmetic circuit 6; a gain control circuit 9 for supplying correction quantity in response to the maximum value of difference in the maximum value detecting circuit 8; and a multiplication circuit 10 for multiplying an output of the LUT memory 7 by an output of the gain control circuit 9 and sending a gradation correcting signal.
Operation of the gradation correcting apparatus will now be explained with reference to FIG. 2a-e, in which each characteristic graph shows a step in the consecutive conversion of a brightness signal within the gradation correcting apparatus. For purposes of this explanation, it is assumed that an input brightness signal is valued in 256 gradations as expressed by an 8-bit binary scale.
First, a brightness distribution of the input brightness signal a is obtained. This brightness distribution is obtained through a horizontal sweep which comprises a train of pixels. The brightness of each pixel or each sampled pixel is measured and classified into 256 brightness grades. The frequency of each brightness grade may be counted and presented in the form of a histogram as shown in FIG. 2(a). This brightness distribution is stored in the histogram memory 5. The contents of the memory 5 are cleared off and the memory 5 is set to zero once every one vertical scanning duration (or integer multiple thereof).
Next, the LUT arithmetic circuit 6 (which may include, for example, an arithmetic logic unit or a microprocessor) conducts the accumulative addition of frequency in the histogram, calculates the normalization coefficient such that the maximum accumulative frequency as obtained in the last accumulative addition is equal to the maximum value 256, and multiplies each accumulative frequency by the multiplication coefficient, as shown in FIG. 2(b).
The result b of the preceding procedure is stored in the LUT memory 7.
Whenever an input brightness signal a is received by the LUT memory 7, a correction signal c is transmitted therefrom. The value of the correction signal c is given by the algorithmic operation such that correction value equals (normalized accumulative frequency (dimensionless) minus input brightness value (dimensionless)), thus leading to the characteristic graph as shown in FIG. 2(c). This graph provides for correction of picture brightness such that lower brightness level are reduced while higher brightness level are increased. In other words, this graph provides for enhanced contrast.
Put another way, c=b (the accumulative addition of frequency)--45.degree. line of accumulative addition of frequency. Alternatively, correction signal c is the difference between an input image and an image having ideal gradation.
By means of the maximum value detecting circuit 8, the maximum value of difference d between adjacent normalized accumulative frequencies as indicated in FIG. 2(b) is detected, and sent to the gain control circuit 9.
Next, by means of the gain control circuit 9, the maximum value of difference d is converted to a value of a gain correction signal e according to the characteristic graph shown in FIG. 2(d), which is obtained by expertise of this art. The more concentrated the input brightness values are in a certain brightness level, the larger the maximum value of difference d becomes. The graph shown in FIG. 2(d) indicates that in the case of concentrated input brightness values, the value of the gain correction signal e has to be reduced.
The gain correction signal e transmitted from the gain control circuit 9 determines the resultant gain of the multiplication circuit 10 as the value of the gain correction signal divided by the number 256. For example, when the value of the gain correction value is 128, the gain is 1/2, thus the value of the outgoing signal, i.e., a gradation correction signal f, becomes half the value of the incoming signal, i.e., the correction signal c. The plot of the value of the gradation correction signal f against the value of the brightness signal a is shown in FIG. 2(e). Correction signal f is added to the input brightness signal a at the addition circuit 11, resulting in the corrected brightness signal g.
The gain control circuit 9 includes a read only memory (ROM), and the characteristic as shown in FIG. 2(d) is written into the ROM.
The gain control circuit 9 transmits the gain correction signal e upon receipt of the maximum value of difference d.
Unfortunately, gain control circuits incorporating a ROM are complex and large. This results in a large PC board and a high cost.